Adolescents with Type 1 diabetes mellitus often struggle to maintain adequate treatment adherence and diabetic control, leading to preventable hospitalizations and emergency room visits. Numerous cross-sectional and prospective studies show that family communication and conflict resolution skills are important influences on adolescents' diabetic control, treatment adherence and psychological adjustment. Empirical validation of psychological interventions targeting these processes could reduce excess health care costs and risks of diabetic complications. In the parent grant, Behavioral Family Systems Therapy (BFST; Robin & Foster, 1989) yielded improvements in family communication skills and parent-adolescent relationships, but it had weaker and less durable effects on treatment adherence and diabetic control. This competing continuation application relies on extensive preliminary data and clinical experience with BFST and on the results of others' investigations, to formulate refinements to BFST designed to maximize its impact on diabetes treatment adherence and metabolic control. These include required targeting of behavioral barriers to adherence and diabetic control for every family, lengthening treatment from 3 to 6 months, and incorporating several treatment components shown to be effective in other studies. A randomized, controlled trial of this refined BFST intervention will be compared to standard medical therapy or participation in a diabetes educational support group using the following measures: family communication, parent-adolescent relationships, adolescent psychological adjustment, treatment adherence, diabetic control and health care use. Predictors of treatment outcome will be analyzed and the clinical significance will be evaluated, social validity and maintenance of treatment effects will be followed-up over 6- and 12-month intervals. These results could influence the clinical practice of diabetes management and health care policy regarding adolescents with diabetes and other chronic diseases.